Electropolishing of gold alloys



United States Patent ELECTROPOLISHING OF GOLD ALLOYS Johannes Fischer,Hanan, Germany, assignor to Deutsche Goldand Silber-Scheideanstaltvormals Roessler, Frankfurt am Main, Germany N0 Drawing. ApplicationJune 18, 1954, Serial No. 437,856

Claims priority, appiication Germany June 27, 1953 6 Claims. (Cl.204140.5)

The present invention relates to improvements in the electropolishing ofgold alloys, and more particularly is directed to a modification of theprocess for electropolishing and deburring gold and gold alloysdescribed in the copending application of Marianne Reichert, Serial No.310,540, filed September 19, 1952 and the corresponding German PatentNo. 888,492.

In the process described in application Serial No. 310,- 540, gold andgold alloys of a fineness of at least 8 carats are anodicallyelectropolished and deburred in an acidic aqueous electrolytic bathcontaining at least g. of thiourea, preferably to 60 g., per liter, attemperatures up to 100 C. The baths used for such electropolishing arepredominantly aqueous and contain only a minor pro portion of acid forthe acidification thereof. Both or ganic and inorganic acids can beemployed for such acidification, such as, for example, sulfuric acid,phosphoric acid, perchloric acid, phenol sulfonic acid, tartaric acidand the like, or mixtures thereof. Especially good results have beenobtained using minor proportions of sulfuric acid and of mixtures ofsulfuric and tartaric acid.

The current densities employed for such process are sufficiently high tocause formation of a film upon the surfaces being anodically treated.

In practicing the process described in Serial No. 310,- 540, it wasfound that, while excellent electropolishing and deburring effects couldbe achieved thereby, it was in some instances necessary to employrelatively high current densities to achieve the optimum polishingelfects, which lead to an undesirably high removal of gold from thesurfaces being polished. Furthermore, the baths had insufiicientthrowing power for some purposes.

It is an object of the present invention to provide a process forelectropolishing gold alloys which will produce optimum polishingeifects at substantially lower current densities and potentials than arerequired with the baths described in copending application Serial No.310,540.

It is a further object of the invention to provide electropolishingbaths for gold alloys having greater elasticity in their use and rangeof polishing effects obtainable, as well as having improved throwingpower.

In accordance with the invention, it was found that these objects couldbe attained by incorporating minor proportions of metal compoundscontaining ions of at least one of the metals contained in the alloy tobe polished which is less noble than the gold, that is above gold in theelectromotive series, in the acidic thiourea containing bath employedfor the electropolishing treatment according to application Serial No.310,540.

Preferably, such metal ions are introduced into the electropolishingbath in the form of their salts. It was found that especially goodresults are obtained when compounds of the least noble metal containedin the alloy are employed, that is, the metal with the largest normalnegative potential.

The quantity of such metal compound incorporated in the electropolishingbath depends upon concentration of Cir 2,712,525 Patented July 5, 1955the preparation of the electropolishing baths containing an addition ofcopper, either solutions of thiourea and copper sulfate can be admixedor these can be admixed in solid form and then simultaneously dissolvedin the baths. It has been found particularly expedient to incorporatecopper in the electropolishing baths by way of the copper salt ofthiourea.

It was furthermore found that the addition of the metal salts to theanodizing baths upon reaction with the thiourea imparted the necessaryacid character to the baths so that it was unnecessary to add furtheracidifying agents thereto. Consequently, in practicing the presentinvention, it is not necessary that such baths be specially acidifiedwith other acids, such as, sulfuric and tartaric acids.

It has also been found advantageous for the polishing effect obtained tomove the article being electropolished periodically in the baths afterthe surface roughnesses originally present have been substantiallyevened out. Evidently, such movement partially dissolves the anode filmformed during the anodic treatment, so that the electric current canagain act upon such locations where the anode film has been dissolved.

The following examples will serve to illustrate several modifications ofthe process according to the invention and the baths employabletherefor.

Example 1 An acidic aqueous bath was prepared by dissolving 50 g. ofthiourea and g. of copper sulfate in 1000 cc. of water, boiling thesolution until the precipitated sulfur had been converted to a form inwhich it was easily filtered and then filtering off the sulfur. The thusprepared bath was suitable for the electropolishing of the usual goldalloys containing at least 33.3% gold, and silver and copper in additionto the gold. A typical gold alloy of this character, for example, wasone containing 83.3% Au, 10% Ag and 6.7% Cu. For the anodicelectropolishing of the gold alloys, a current density of 6 A./dm. at 3volts was employed. The duration of the anodic treatment to produce agood polished effect varied from 5 to 10 minutes, depending upon theoriginal roughness of the articles treated.

Example 2 An acidic aqueous bath was prepared by dissolving g. ofthiourea and g. of thiourea-copper sulfate in 1000 cc. of hot Water. Thethiourea-copper sulfate was obtained by reacting thiourea and cuproussulfate according to Beilstein, vol. III, page 186. The equation for thereaction is as follows:

This bath gave excellent electropolishing with gold-silvercopper alloysunder the conditions specified in Example 1, except that the bathtemperature was C.

Good results were also obtained by replacing the thiourea-copper sulfatecompound with the acid thiourea salt of acid copper sulfate of theformula obtained by reacting two mols of thiourea with acid coppersulfate.

Example 3 An acidic aqueous bath was prepared by dissolving 75 a g. ofthiourea and 10 g. of silver nitrate in 1000 cc. of water, boiling thesolution and filtering off the precipitated sulgfur.

This bath was especially adapted for anodically electropolishing copperfree gold-silver alloys, even, those produced electrolytically, atcurrent densities of 5 A./dm.

at 3 volts. For polishing electrolytically produced gold- E silveralloys the duration of the.electropolishing was about 10 minutes.

Example 4 An acidic aqueous bath was prepared by dissolving 50 g. ofthiourea and 15 g. of nickel sulfate in 1000 cc. of

water, boiling such solution, and filtering to remove precipitatedsulfur. This bath was especially adapted for electropolishing theso-called white gold alloys in which nickel is the least noble alloyingcomponent. White golds usually contain 33% to 75% Au and the remainderessentially nickel with minor amounts of silver and copper.

In electropolishing such white gold alloys, a current density of 6A./clrn. was employed, with the bath maintained at a temperature of 60C. The time required for the anodic electropolishing was about 5 to 10minutes.

I claim: 1. A process for polishing the surfaces of gold alloys of afineness of at least 8 carats which comprises anodicallyelectropolishing such surfaces in an aqueous acidic bath containing atleast 5 g. of thiourea per liter and to' which 0.03 to 0.3 mols of asalt furnishing ions contain- 3 ing at least one of the metals containedin the gold alloy to be polished which is less noble than gold has beenadded from an external source, at a current density causing theformation of a film upon the surfaces treated.

2. A process in accordance with claim 1 in which said salt is a metalsalt which furnishes ions of the least noble '2 alloying component inthe gold alloy.

faces being electropolished are periodically moved in the bath.

References Cited in the file of this patent FOREIGN PATENTS 888,492Germany Sept. 3, 1953

1. A PROCESS FOR POLISHING THE SURFACES OF GOLD ALLOYS OF A FINENESS OF AT LEAST 8 CARATS WHICH COMPRISES ANODICALLY ELECTROPOLISHING SUCH SURFACES IN AN AQUEOUS ACIDIC BATH CONTAINING AT LEAST 5 G. OF THIOUREA PER LITER AND TO WHICH 0.03 TO 0.3 MOLS OF A SALT FURNISHING IONS CONTAINING AT LEAST ONE OF THE METALS CONTAINED IN THE GOLD ALLOY TO BE POLISHED WHICH IS LESS NOBLE THAN GOLD HAS BEEN ADDED FROM AN EXTERNAL SOURCE, AT A CURRENT DENSITY CAUSING THE FORMATION OF A FILM UPON THE SURFACES TREATED. 